A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that radiation with a wavelength of less than 10 nm could be used, for example 6.7 nm or 6.8 nm. In the context of lithography, wavelengths of less than 10 nm are sometimes referred to as ‘beyond EUV’.
Extreme ultraviolet radiation and beyond EUV radiation may be produced using a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g. tin), or by directing a laser at a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which is collected using a collector such as a mirrored grazing incidence collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam.
In addition to extreme ultraviolet radiation, the plasma produces contamination in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The contamination is projected, together with the extreme ultraviolet radiation, towards the collector and may cause damage to the grazing incidence collector.
It is therefore desirable to prevent contamination from entering and damaging the collector or to reduce the amount of contamination that enters and damages the collector.
An apparatus that may be used to prevent contamination from entering and damaging the collector, or to reduce the amount of contamination that enters and damages the collector, is a foil trap. A foil trap comprises a plurality of planar foil members that are located between an emission point of a radiation source, and the collector. The planar foil members are aligned such that they are radially aligned with respect to the emission point, so that the planar foil members obscure as little radiation as possible that is emitted from the emission point. In other words, the planar foil members are oriented such that they are parallel to the direction of travel of the radiation. The foil trap may be used to trap particles, the particles impinging against and/or being caught by the planar foil members. The functionality of the foil trap may be improved by rotating the foil trap, and/or by introducing a buffer gas in the vicinity of (e.g. in-between) adjacent planar foil members. Alternatively, a buffer gas may be used in conjunction with a foil trap in a configuration where the foil trap does not act primarily to trap particles but to cool the buffer gas. For example, in such a configuration, the distance between the planar foil members may be larger.
When used in conjunction with an EUV radiation source, the heat load on the planar foil members is high. The high heat load can lead to degradation, damage or destruction of the planar foil members. Centrifugal forces associated with the rotation of the foil trap can make the degradation, damage or destruction of the planar foil members more likely. When a buffer gas is used in conjunction with a foil trap, the heat load on the buffer gas will also be high, and may be so high as to be unacceptable.
It is therefore desirable to reduce the heat load on a foil trap and/or a buffer gas used in conjunction with such a foil trap.